The invention relates to the field of recording data on support media readable optically by means of a concentrated radiation and relates more particularly to an optical writing-reading head for an optical disk and an optical device associated with such a head.
In the case of writing and/or reading an optical type disk, it is necessary:
to have access to a zone of the disk, PA1 to remain in this zone with good precision not only radially but also perpendicularly to the surface of the disk (focussing).
In the devices of the prior art an "optical head" fulfils these latter two functions; it is generally formed of an actuator or motor of the "loud-speaker coil" type providing the focussing movement perpendicularly to the face of the disk and an actuator providing the radial track following movement, formed either by an oscillating mirror or by a translation device able to be fixed in the mobile part of the focussing actuator.
Access is obtained by moving the whole of the "optical head" either with a screw system and a DC motor or stepper motor, or with a motor and band system. The technique of the linear motor used in some magnetic disks may also be used.
It is advantageous to be able to use the access actuator for providing at least partially the radial tracking while keeping for this actuator a reduced space occupancy and a control power which is also reduced.
The "optical heads" used conventionally comprise one or more magnetic circuits providing the focussing movements and radial tracking; that results in a relatively high mass for these "optical heads" (of the order of 20 to 40 g).
If it is desired to obtain at least a partial correction of the radial tracking with the access actuator, this latter can very certainly only be of the linear motor type. The mass of the optical head imposes the dimensions and the mass of the linear access motor and consequently the control power of the access actuator.
So as to minimize the dimensions and also consequently the control power of the access actuator, it is apparent that the mass of the "optical head" should be reduced. One solution consists in making the magnetic circuits of the "optical head" fixed and so in coupling to the mobile lens only coils interacting with a fixed field.
However, two coils seem to be duplicated: on the one hand, the coil of the access actuator which provides access and a partial correction of the track deviation and, on the other hand, the coil of the radial tracking actuator. For this the invention consists first of all in eliminating the tracking coil, the access coil then having the two access and radial tracking functions.
Such a device may operate but its performances are limited by the solid friction, essentially non linear, introduced by the guiding of the access coil, whether this guiding is effected by rolling means (rollers or ball bearings) or simple sliding means.
With either rolling or friction guiding, effected directly on the central magnetic core, with such a device a tracking precision of the order of a few micrometers may be obtained. But in an optical type disk, the required precision is of the order of 0.1 micrometer.
Thus, a device of the prior art, described in European patent application published under No. 0 012 650, relates to a device providing vertical and radial positional control by moving a very small weight mobile assembly, these two controls being provided by currents proportional to the movements to be obtained. For that, conducting elements through which the control currents flow are moved in a fixed magnetic field, orthogonal to the radius and extending over the whole effective length of the radius of the disk, and moving in this magnetic field; the reading lens being made integral with these mobile elements.
However, this prior art device is provided with sliding parts in the air-gap of its magnetic circuit and thus presents limitations related to the slide friction and described above.
Thus the correct operation of a combined access and tracking device is coupled essentially to guiding thereof and it is the guiding device for overcoming this serious drawback of the prior art which is the principal object of the invention. In the device of the invention, for providing this guiding, solid friction is kept, guiding by simple sliding is chosen and between the sliding means and the body of the access and radial tracking actuator are inserted elements having great flexibility. Moreover, for this guiding, pairs of materials are used having a minimum friction coefficient.
Moreover, with the focussing actuator fixed on the radial actuator, besides the expected mechanical advantages, certain optical reaction defects may be avoided which are inherent in conventional optical heads; in fact, whether it is a system with radial oscillating mirror or a system with radial translation actuator fixed on the focussing actuator, a movement of the optical beam cannot be completely avoided causing either a diaphragm effect of the pupil of the lens, or a movement or a deformation of the optical return spot.
On the contrary, in the case of a focussing actuator fixed on the radial tracking actuator, these effects are non existent and a radial tracking sensor system, for example of the "push-pull" type, may be used.